One of the issues confronting formulators of inkjet ink sets comprising black and color (e.g., cyan, yellow, magenta) is that of black-to-color bleed. This has been an issue earlier for dye-based colorants and, more recently, for pigment-based colorants.
The term “bleed” as used herein, is defined as follows: When inks of two different colors are printed next to each other, it is desired that the border between the two colors be dean and free from the invasion of one color into the other. When one color does invade into the other, the border between the two colors becomes ragged, and this is bleed. This is in contradistinction to uses of the term in the prior art, which often defines “bleed” in the context of ink of a single color following the fibers of the paper.
The term “halo” applies to the printing defect that occurs in black areas surrounded by color ink. In some cases, a distinct light-gray zone is seen where the black is adjacent to color. The defect is normally seen within 2 to 3 mm from the line dividing the black and color areas and occurs on the black side.
Finally, “mottle” is defined as a color or black non-uniformity on top of the paper, with some areas darker than others.
The present teachings specifically address the case where the color inks are dye-based and the black inks are pigment-based. This approach turns out to deliver the best performance for ink-jet printers overall, combining the good text quality and business graphics of the pigmented black inks with the bright colors of the dye-based color inks. Dye-based systems, in which the dye colorant is soluble in water, have different considerations than pigment-based systems, in which the pigment colorant is insoluble in water and must be rendered dispersible, such as by the use of polymeric dispersants or by self-dispersing the pigment.
Adding polymers to pigmented systems is well known in the art. Polymers tend to stabilize the dispersion, as well as improve such attributes as the black-to-color bleed, optical density of the black, halo, and mottle. For example, pigmented inks have been disclosed that contain AB and BAB block-copolymers, where the blocks are made out of various monomers of acrylic family. As another example, a relatively high molecular weight (>10,000) polysaccharide such as alginic acid or carrageenan has been used. As another example, the use of polymeric resins, in particular, styrene-acrylic copolymers in pigmented inks is known. Still further, inks containing a carboxylic salt polymer are known. Also, inks containing a copolymer of a hydrophobic α,β-unsaturated ethylene monomer, and a hydrophilic monomer having a plurality of COOM groups (where M is hydrogen, alkali metal, or an ammonium ion) are known. Specifically, the hydrophobic monomer can be styrene and hydrophilic monomer can be maleic anhydride. In addition, an ink-jet ink containing two dispersants: one containing a sulfonic acid salt/ester, and the other containing a carboxylic acid salt, within a specific range of molecular weights has been described. Finally, a number of polymers for the bleed and halo control are described, most of them being of the acrylic type.
It has been known in the art that introducing chemical reactivity between the color and black inks brings many positive features to ink-jet performance, such as color-to-black bleed control and decreasing the halo. Also, it has been discovered that if reactive color inks are used for underprinting under the black areas, the optical density of the black is improved. The interaction between the polymeric dispersant and polyvalent metal ions in the color inks has been described for controlling bleed/halo/mottle. Alternatively, the reaction with the polymer can be triggered by organic acids in the color inks.
Aside from polymeric dispersants, there is another approach for preparing colloidal pigment dispersions, called self-dispersion. It typically involves attaching solubilizing groups to the pigment. There are several methods known in the art that enable the attachment of a solubilizing ionic group to the surface of a pigment particle. For example, a method of attaching an ionic group to the carbon surface is based on the reactions of diazonium salts. On the other hand, a reaction by oxidation of the surface of the pigment particles may be performed using ozone.
Developing black-to-color bleed control mechanisms for self-dispersed black pigment inks, such as those available from Cabot Corp., is typically harder than for conventionally dispersed black inks (e.g., using polymeric pigment dispersants) because the self-dispersed pigments are highly electrostatically stabilized. In addition, there is no polymeric dispersant which, when reacted, rapidly builds viscosity and limits ink migration. As an example, a series of polymers of the acrylic type have been used to improve the color-to-bleed and halo of the black pigmented inks.
However, adding polymeric dispersants to the pigment-based black inks in many cases leads to nozzle clogging and other reliability issues. Therefore, a need remains in further improving the polymeric dispersants. Also, a need remains in finding the polymers that can provide the best reactivity with the reactive components in the dye-based color inks.